Reactive liquid phase sintering of YBa 2 Cu 3 O 7-x superconducting thin films: Part II. Sintering mechanism and film pr
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Reactive liquid phase sintering of YBa2Cu3O7_x superconducting thin films: Part II. Sintering mechanism and film properties Peir-Yung Chu and Relva C. Buchanan Department of Materials Science and Engineering, University of Cincinnati, Cincinnati, Ohio 45221 (Received 23 July 1993; accepted 22 November 1993)
Reactive liquid phase sintering of the YBCO films prepared from carboxylate solution precursors was investigated. A liquid phase was formed by the BaO-CuO binary compound at 620 °C, which facilitated the crystallization and growth of the YBCO grains. Strong capillary forces associated with the liquid phase sintering resulted in a dense, highly oriented microstructure. The YBCO films processed by this reactive liquid phase process showed r c(zero) = 90.2 K and Jc = 1.3 X 105 A/cm 2 at 77 K. The (006) rocking curve measurement indicated excellent grain orientation with a FWHM better than 0.2°. A secondary ion mass spectroscopy (SIMS) study showed uniform film composition, and the film/substrate interaction zone was limited to —0.1 /xm. Average film roughness was about 32 nm.
I. INTRODUCTION A common characteristic of all forms of sintering is a reduction in surface area and an increase in strength of the samples. Solid state sintering, which involves only solid phases, is traditionally used for ceramic powder processing. When one or more liquid phases coexist with solid powders during the sintering process, liquid phase sintering takes place. There are three stages in the classic liquid phase sintering process. Initially, the powder compact is heated to a temperature where a liquid phase forms. The liquid phase spreads and wets the particles, which causes particle rearrangement through capillary forces. The second stage is the solution-reprecipitation stage, where atomic diffusion, grain growth, and shape accommodation take place. The last stage is referred to as solid-state controlled sintering, where densification is slow since a solid skeleton is already present.1"3 Important parameters that critically impact liquid phase sintering include surface energy, solubility, capillarity, viscous flow, and segregation of impurities. The major advantages of liquid phase sintering include reduced sintering temperatures, rapid densification, higher sintered densities, and microstructures that can be tailored to give desired properties.3 Liquid phase sintering plays an important role in the processing of abrasives, ferroelectric capacitors, ferrite magnets, and high temperature ceramics including carbides.2'4 Recently, liquid phase sintering processes (such as melt growth) have been widely used to prepare YBa 2 Cu 3 07_ j: (YBC0) superconductors to achieve higher current densities.5'6 In this method, the YBCO samples are fired above the melting temperature (1015 °C in air) and then quenched to lower temperatures; this results in a highly oriented microstructure and large grains. Although this process does not follow exactly the conventional liquid phase 844 http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 4, Apr
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